The prison population in the U.S. is now approaching the 2 million mark. With growing prison overcrowding and resultant inmate tensions, the guards and correctional officers are increasingly exposed to stab attacks by violent criminals using a variety of home made thrusting weapons such as shanks, ice picks, knives, sharpened nails or other objects. As these attacks can inflict life-threatening wounds, many guards are now wearing protective suits designed to stop penetration of such weapons. The suits are made with multiple woven fabric layers using high performance, high tenacity yarns.
A typical fabric used is woven with one of the above yarns in a density of 70 picks.times.70 warp ends/inch. The fabric is coated with a special resin to resist and retard penetration of an ice pick point by preventing the weave components from shifting. About 27 layers or plies of fabric are combined to provide a protective, stab resistant shield placed inside the suit. The suits are tested using the current standard, which stipulates resistance to an impact valve of 81.1 foot-pounds using a 7 inch long pick in a diameter of 0.0163 inch with a 15:1 taper and steel hardness of 42 c, as per the State of California body armor specifications.
Circular weft knit fabrics have also been introduced into this field. They are made on interlock or double knit machines. These fabrics rely on heavy resin impregnation to stabilize or "freeze" the loop components in order to enhance resistance to penetration by a pick or other weapon.
One of the main disadvantages of woven fabrics in offering protection from thrust or stab weapons is the relative ease with which the weft and warp threads slide on each other as the weapon's point impinges on them, pushing them aside, and leading to effective penetration. All woven structures are held together by friction existing between its components and have to use various resins to immobilize them and preserve the fabric integrity as it is impacted by a thrusting point. This makes the fabric heavy, stiff and almost impenetrable to air, leading to wearer discomfort.
Circular knit fabrics, while more permeable to air than woven ones, are inherently unstable due to their residual elasticity, which must be eliminated with the application of a heavy resin coating. Furthermore, circular knit fabrics cannot be made very tight in order to produce a high thread density per unit area. This is because of their rather open loop structure.
For fabrics to be truly effective in resisting point penetrations, it is essential to have a high density of yarn crossings, firmly anchored in the matrix of its structure, and without relying on friction or resin impregnation. Warp knit fabrics satisfy these requirements better than wovens or circular knits.